The present invention generally pertains to MOSFET switching devices and is particularly directed to cancelling the charge spikes occurring at the source and drain of the MOSFET when the conduction state of the MOSFET is changed.
The basic structure of a typical MOSFET switching device is shown in FIG. 1. This device is an enhancement mode MOSFET (metal-oxide-semiconductor field effect transistor). The device includes a silicon substrate 10 of either p-type or n-type material containing diffusions 11, 12 of the oppositive p-type or n-type material. A silicon dioxide layer 14 covers the substrate 10 and the diffusions 11 and 12. Conductive material contacts 15, 16, 17 are formed in the silicon dioxide layer 14. The diffusion 11 is the source and the diffusion 12 is the drain. Contact 15 extends through the silicon dioxide layer 14 and makes contact with the source 11. The source contact 15 also is electrically connected to the silicon substrate 10 by a conductor 13. Contact 16 extends through the silicon dioxide layer 14 and makes contact with the drain 12. Contact 17 contacts the gate 19. The gate 19 is a thin zone within the silicon oxide layer 14 beneath the contact 17.
When a voltage of a given polarity with respect to the voltage applied to the source contact 15 is applied to the gate contact 17, an electric field is established through the thin silicon dioxide gate 19 into a channel 20 which extends between the source 11 and drain 12 and thereby enables current to flow from the source 11 to the drain 12. When this occurs, the MOSFET is said to be rendered conductive.
However, when the electric field is established in the channel 20 an electric field is also established within the thin silicon dioxide gate 19 between the gate contact 17 and the source 11, and between the gate 19 and the drain 12.
Thus, when the MOSFET is rendered conductive, charge spikes of one polarity appear at its source and drain; and when the MOSFET is rendered non-conductive, charge spikes of the opposite polarity appear at its source and drain.
These charge spikes which occur at the source and drain when the conduction state of the MOSFET is changed can be particularly troublesome in some circuit applications for MOSFET switching devices. For example, when a MOSFET switching device is connected to a capacitance in a sample and hold circuit, the capacitance is charged to a given voltage when the switch is closed (rendered conductive). It is desired that when the MOSFET switch is opened (rendered non-conductive), that the charged voltage of this capacitance remain unchanged and not be affected by the charge spikes occurring at the source or drain of the MOSFET.